docs: def set_XRef(self, X=None, indtX=None, indtXlamb=None):
          """ Reset the reference X
  
          Useful if to replace channel indices by a time-vraying quantity
          e.g.: distance to the magnetic axis
  
          """
          out = self._checkformat_inputs_XRef(X=X, indtX=indtX,
                                              indXlamb=indtXlamb)
          X, nnch, indtX, indXlamb, indtXlamb = out
          self._ddataRef['X'] = X
          self._ddataRef['nnch'] = nnch
          self._ddataRef['indtX'] = indtX
          self._ddataRef['indtXlamb'] = indtXlamb
          self._ddata['uptodate'] = False